Monitoring 135Xe after the shutdown of a research reactor using gamma-ray spectroscopy

135Xe, a high-yield fission product with a large neutron absorption cross section, can greatly influence the reactivity of a reactor, posing challenges to reactor operation and safety if the transient 135 Xe concentration is not taken into account. To date, 135Xe has only been monitored indirectly with neutron flux monitors, studied from off-gas, or studied in simulation. In this study, we show that 135Xe in a zero-power research reactor can be directly monitored after shutdown, using gamma spectroscopy. We use an HPGe gamma-ray spectrometer in the beam port of the CROCUS research reactor and measure the 135Xe and 135I signature gamma-ray emissions (250 keV and 1260 keV, respectively) after shutting down the reactor. We directly measure the transient behavior of 135Xe and 135I, the commonly known “Xenon Pit” corresponding to maximum neutron absorption, following reactor shutdown. The expected 135Xe peak time with known half-life values is 11.13 h, and we calculate 11.15 ± 0.22 h when fitting the measured data with free half-life parameters. We provide results for code validation, especially useful for reactor shutdown and accident scenarios, as well as reactivity margin predictors.